Composite metal article



Feb. 24, 1970 J. WINTER COMPOSITE METAL ARTICLE Original Filed Ilarch30, 1966 INVENTOR. JOSEPH WINTER #TTORNEY United States Patent 3,496,625COMPOSITE METAL ARTICLE Joseph Winter, New Haven, Conn., assignor toOlin Mathieson Chemical Corporation, a corporation of VirginiaApplication Mar. 30, 1966, Ser. No. 538,697, which is acontinuation-in-part of application Ser. No. 520,404, Jan. 13, 1966,which in turn is a continuation-in-part of application Ser. No. 229,262,Oct. 2, 1962. Divided and this application Jan. 16, 1968, Ser. No.719,811

Int. Cl. B21b 3/00; 1332b 15/20 US. Cl. 29199 4 Claims ABSTRACT OF THEDISCLOSURE The disclosure teaches a process for obtaining compositemetal articles and the article obtained thereby wherein a first metalcomponent has a recrystallization temperature under 500 F. and a secondmetal component has a recrystallization temperature at least 100 F.higher than the first metal component, with the components being rolledtogether in direct, face-to-face contact at a speed of at least 20 feetper minute in one pass at a reduction between 40 and 90%, with the firstcomponent only being recrystallized, thereby forming an integratedcomposite article.

This application is a division of copending application Ser. No.538,697, filed Mar. 30, 1966, now abandoned, which application is acontinuation-in-part of US. patent application Ser. No. 520,404, filedJan. 13, 1966, now abandoned which in turn is a continuation-in-part ofUS. patent application Ser. No. 229,262, filed Oct. 2, 1962, nowabandoned.

The present invention relates to composite metal articles. Moreparticularly, the present invention resides in composite metal articleswherein at least one component is a relatively soft metal, such ascopper and at least one component is a relatively harder metal and tothe process whereby said composites are obtained.

A particularly preferred embodiment of the present invention is thosecomposites having a core of commercial purity copper clad on one or bothsides with cupronickel, a copper base alloy containing about 75% copperand 25% nickel. This composite finds particular utility in the newUnited States coinage.

Composite articles having a dissimilar core and cladding and highlydesirable commercially due to the fact that the beneficialcharacteristics of the core and cladding materials may be obtained inone composite article. In a single alloy frequently many propertiescannot be greatly modified by alloying or thermal treatments, forexample, such properties as modulus of elasticity, color, density, andstrength in combination with high thermal or electrical conductivity.However, by forming composites apparent properties of the cladding canbe generated while retaining the bulk properties of the core material.In this manner one can often obtain greatly modified and highlydesirable properties over the single alloy.

For example, copper has the advantage of high conductivity and coldformability. By forming copper composites one can retain these desirableproperties while generating properties of the cladding, such as wearresistance, color, oxidation or tarnish resistance and fine finishing,i.e., surface quality.

Various uses to which composite copper articles may be put are asfollows: high conductivity, high strength springs; highly efficientelectrical contactors; hardware involving extensive cold forming.

However, the preparation of composite copper articles presents numerouspractical problems. It has been dif- ICC ficult to produce asatisfactory composite article wherein at least one of the components iscopper or a copper base alloy due to the tendency of one of thecomponents to form a layer of brittle intermetallic compounds at theinterface with the copper component. This layer may form at moderatetemperatures or at elevated temperatures. The brittle layer ofintermetallic compounds which so forms may shatter readily on flexing ofthe composite material, thus clearly limiting the usefulness of thecomposite.

In addition, it is frequently difiicult to .obtain a well bondedcomposite article which will withstand normal expected use.

Copper presents further and peculiar problems in the formation ofcomposites due to the tendency of copper to oxidize at the moderate orelevated temperatures necessary for hot rolling. This oxidation willproduce an interference oxide layer which inhibits bonding. Some copperoxides are particularly diflicult as they form a strongly adherent,plastic layer which greatly interferes with bonding. Other oxides, onthe .other hand, tend to break up on hot rolling, for example, aluminumoxides and iron oxides, thus not forming an interference layer.

The tendency of copper to form this peculiar oxide has necessitatedspecial and expensive treatment conditions in order to form coppercomposites.

One method of forming copper composites in order to overcome thesedifficulties is to form a partial bond by cold rolling followed bysubsequent diffusion anneals. This is an expensive process and thediffusion anneals tend to degrade the properties of the composite.

One highly effective method for overcoming the foregoing disadvantagesis found in co-pending application Ser. No. 229,262. This method is alsodescribed in copending application Ser. No. 454,182, filed May 7, 1965.In general, the foregoing co-pending aplications critically controlcertain variables and thereby obtain a highly convenient and expeditiousprocess which results in a greatly improved composite article. In theseprocesses the copper base alloy core only is heated to an elevatedtemperature followed by rolling together the heated core and the coldcladding at high speed of at least feet per minute in one pass at aspecified reduction with the core and cladding coming together for thefirst time in the bite of the rolls, with the cladding contacting theroll prior to contacting the core.

The processes described in the foregoing applications readily obtain anexcellent bond due to the therein described critical conditions and dueto the high temperature of the core with respect to the cladding. Thisresults in an exaggerated difference in ductility between the core andcladding. The processes of said co-pending applications also result inturbulence at the interface of the core and cladding, which turbulencepromotes bonding between the components. The difference in ductility andgenerated turbulence is suflicient to overcome oxide build up bybreaking up the oxide.

It would be highly desirable if a process could be devised which wouldachieve the improved results of said co-pending applications and resultin a convenient, commercially feasible and expeditious process forbonding this type of material. It would be highly desirable if thisprocess could be devised without the practical difficulties attendantupon the use of elevated temperatures which characterizes the processesof sad co-pending applications. It would be highly desirable if aprocess were devised which enables the use of lower temperatures.

Accordingly, it is a principal object of the present invention toprovide new and improved composite articles and convenient andexpeditious processes whereby said composite articles are obtained.

It is a further object of the present invention to provide a process andarticle as aforesaid whereby the articles are characterized by havinghigh strengths, excellent physical properties and suitable for a widevariety of uses.

It is a further object of the present invention to provide a process andarticle as aforesaid wherein one component of the composite isrelatively soft with respect to the other, in particular, compositearticles having a copper core and a dissimilar cladding.

It is a still further object of the present invention to provide aprocess and article as aforesaid which ever comes the numerouslimitations and disadvantages attendant upon the formation ofconventional composite articles.

Still further objects and advantages of the present invention willappear from the ensuing discussion.

In accordance with the present invention, it has noW been found that theforegoing objects and advantages may be readily obtained. The process ofthe present invention readily overcomes the disadvantages of the art andachieves a simple and convenient process for obtaining a highly usefulcomposite article clad on either one side or both sides. The process ofthe present invention is a process for obtaining a composite articlehaving a metal core, clad with a dissimilar metal which comprises: (A)providing a first metal component of the composite having arecrystallization temperature under 500 F., preferably copper or acopper base alloy; (B) providing a second metal component of thecomposite having a recrystallization temperature at least 100 F., higherthan said first component; (C) rolling together said components indirect, face-to-face contact at a speed of at least 20 feet per minutein one pass at a reduction between 40 and 90%, said reduction beingsuflicient to cause said first component only to recrystallize, therebyforming an integrated composite article. This recrystallization mayoccur either in the bite of he rolls or immediately upon exit from therolls.

In the preferred embodiment, the softer component, i.e., the componenthaving a recrystallization temperature under 500 F., is the core and thesecond component is the cladding. It is also preferred that the core andcladding come together for the first time in the bite of the rolls.

By recrystallize it is meant the presence of metallographically visible,equiaxed grains of unstressed material throughout at least 25 percent ofthe surface area being metallographically examined. By recrystallize itis also meant the decrease in hardness of the material beingrecrystallized by at least 25 percent of the difference in hardnessbetween material fully hardened by cold working and material fullythermally softened.

The theory behind the process of the present invention is as follows.The higher the deformation or the reduction of the composite in therolls, the more heat is imparted to the composite, i.e., a rollingreduction of for example 75% will cause the composite to heat up to agreater degree than a rolling reduction of for example 40%. If anappropriate reduction is chosen to cause one of the components torecrystallize by imparting a certain level of heat and residual work tothe composite, than that component which is being recrystallized isundergoing hat rolling and it is getting softer during the rolling thanthe other component, i.e., the component which is not beingrecrystallized is getting relatively harder during the rolling. Thisexaggerated difference in ductility between the core and claddingresults in turbulence at the interface between the core and claddingwhich turbulence promotes bonding.

Naturally, recrystallization is a time and temperature dependentphenomenon. Therefore, recrystallization does not spontaneously occurthroughout the component being recrystallized. Recrystallization occursover a finite p c o time which d p nds upo th percen g roll g Citreduction and the temperature attained in the bite of the rolls. Thehigher the temperature achieved in the bite, the faster the rate ofrecrystallization.

Accordingly, in accordance with the present invention the process forrecrystallization has started in the bite of the rolls, and recognizablerecrystallization has occurred at least within seconds after exit fromthe rolling mill. The exact distance from the mill will depend uponrolling speed.

The resultant composite is characterized as follows: one componentexhibits a microstructure of at least 25 percent, preferably at least 50percent, equiaxed, recrystallized grains, i.e., the micro-grainstructure contains grains spheroidal in nature with the axis of eachgrain being substantially the same. This is caused by the nucleation andgrowth of unrestrained grains which consume strained material. Themicro-grain structure of the other component is grossly elongated in therolling direction, with the length to thickness ratio of the elongatedgrains being at least 2:1. The core and cladding are characterized by adirect, face-to-face bond with the absence of oxide between core andcladding, i.e., little or no apparent oxide between the core andcladding. In addition, the interface between the core and cladding ischaracterized the absence of apparent atomic interdilfusion, with saidinter* face being further characterized by having at least 10% greatersurface contact area than planar sheets.

In the preferred embodiment, upon entering the rolls, an angle isprovided between the core and cladding in excess of 5, and generally inexcess of 10, in order to insure that the core and cladding will notcome together earlier than in the bite of the rolls. This is notessential, however. Generally, the included angle between the core andcladding is between 5 and 22.

In accordance with the preferred embodiment of the present invention,the cladding metal contacts the :roll prior to contacting the core. Thisis true whether the core is to be clad on one side or both sides. On thefront side of the rolls entering side) the cladding and the rolls aretravelling at different linear speeds; whereas, at the exit side theyare going at the same speed due to the reduction in thickness of thecomposite. The difference in travelling speeds between the cladding andthe rolls, cou pled with the pre-contact between the cladding and therolls, coupled with atomic movement inherent in the recrystallization ofthe core, generates a shearing strain to the core-clad interface. Thisshearing strain at the coreclad interface results in turbulent flow ofmetal at the interface which causes an intimate bonding and increasesthe interfacial linear surface of the composite by generally 20% ormore.

It has been found that the simple process delineated above achieves ahighly advantageous composite article, with the interface between thecore and cladding characterized as indicated hereinabove. The compositearticles of the present invention have excellent physical properties,very high bond strengths and the absence of atomic interdiffusionbetween base metal and cladding, which interdiffusion may result in theformation of brittle compounds. The present invention achieves thesesurprising advantages by means of a simple and convenient process andwithout the use of expensive devices such as are frequently employed inthe art.

In accordance with the present invention, at least one component of thecomposite must have a recrystallizatlon temperature under 500 F. andanother component of the composite must have a recrystallizationtemperature at least F. higher than said first component.

By recrystallization temperature it is meant the mini mum temperature atwhich recrystallization of cold worked metal occurs within a specifiedtime; nominally, cold worked infers at least 50% cold work.

In the preferred embodiment, the component with the lowerrecrystallization temperature is the core material. Therefore, thepresent invention will be discussed in more detail with this preferencein mind. It should be understood, however, that either component may beemployed as the core material. Preferably, copper or a copper base alloyis the core material. Representatives copper base alloy materials withrecrystallization temperatures lower than 500 F. are as follows:Commercial purity copper, various high conductivity grades of copperwith conductivities in excess of 5 0% IACS. Alternatively the corematerial may be a low temperature melting material as lead, tin, zinc,aluminum or alloys thereof.

The core material is preferably provided in plate form having athickness less than /2 inch, i.e., the core material may be provided incoils, strip, sheet or the like form.

It is preferred, but not required, in accordance with the presentinvention to mechanically roughen the bonding surfaces of both the coreand the cladding materials in order to assure good surface contact atthe bite of the rolls. For example, the surfaces may be wire brushed orabraded, etc.

The cladding material must have a recrystallization temperature of atleast 100 F. higher than the chosen core material and preferably atleast 300 F. higher than the core material. The differential ofrecrystallization temperatures between the core and cladding are, ofcourse, provided in order to insure that both components do notrecognizably recrystallize as defined above.

The cladding material may be, for example, a copper base alloy or basealloy or lead, tin, nickel, zinc, titanium, iron, silver or aluminum.Naturally, the present invention contemplates cladding the copper coreon both sides with dissimilar metals on each side.

The cladding material should be in plate form less than A; inch inthickness, i.e., the cladding and the core material should be in wroughtform, for example, coils of strip, sheet, etc. Therefore, the resultantcomposite has a thickness less than 0.60 inch where the copper is cladon two sides and less than 0.45 inch where it is clad on one side. Thelowest gage materials, both core and cladding, which can be convenientlyworked with, is on the order of 0.001 inch.

The starting materials, both core and cladding, can be in any temper orcondition, hard or soft as long as the foregoing requirements concerningrecrystallization temperature relationships are met. In the preferredembodiment the core should be in a cold Worked temper and cladding in anannealed temper as this exaggerates the recrystallization temperaturedifferential.

Surface oxides, unless very massive, are generally no handicap to theprocess of the present invention. This is quite surprising and is asignificant advantage of the present invention since conventionalprocessing must remove surface oxides prior to the formation of thecomposite. In fact, conventional processing frequently forms compositesin special atmospheres so that one does not form surface oxides prior tothe formation of the composite. These special precautions are notrequired in the present invention.

It is, however, highly desirable to remove dirt or adhering lubricantfrom the surface of the metal prior to the process of the presentinvention in order to assure good frictional contact between the coreand the cladding materials. Any conventional cleaning processes may bereadily employed, for example, the core and cladding materials may bepassed through a soap or a detergent solution in accordance withconventional procedures. Examples of such cleaning procedures includethe use of commercial alkaline cleaners and solvent cleaners, such ascarbon tetrachloride and trichlorethylene.

It is a surprising feature of the present invention that it desired thecore and cladding may be integrated without heating either one or bothof the components, or at most with only moderate heating of the core.This is a particular advantage of the present invention. For example, ifthe core is commercial purity copper and the cladding is 75:25cupro-nickel, the core recrystallizes at about 375 F. within minutes andthe cladding recrystallizes at about 1000 F. Thus, if the core andcladding are both rolled without heating the core, the one passreduction must be high in order to recrystallize the core as a result ofrolling, i.e., the one pass reduction must be at least 81 to 90%. If itis desired to take a lesser degree of reduction on the composite, thecore should be heated somewhat, for example, from 200 to 275 F. whichnecessitates a rolling reduction of from 70 to in order to recrystallizethe core as a result of rolling. Alternatively, the core may be in acold worked temper, e.g., 3 to 50% cold worked in which caserecrystallization would occur with a 70 to 80% rolling reduction andwithout the use of auxiliary or prior heating, If desired, rollingconditions can be arranged to cause the core materal to attain atemperature in excess of 375 F., thereby causing recrystallization,e.g., control of lubricant, rolling speed, etc. Naturally, due to thehigh recrystallization temperature of the cupro-nickel cladding, thesmall amount of heat imparted to the composite in the rolling step willnot recrystallize the cupro-nickel.

In accordance with the present invention it is preferred that the coreand cladding enter the rolls at an angle so that they come together forthe first time in the bite of the rolls. The materials are rolled at ahigh speed of at least 20 feet per minute in one pass with a reductionbetween 40 and 90%, with these variables in general being governed bythe particular core, by its recrystallization temperature and by itstemperature entering the rolls. In the preferred embodiment thematerials are rolled at 60-3 00 feet per minute with a reduction between70 and The copper core should preferably be maintained between roomtemperature and 275 F. upon entering the rolls, depending upon thedegree of cold work. The greater the cold work, the lower the allowabletemperature.

It is preferred that the core and cladding enter the rolls at an anglein excess of 10 and generally at an included angle of between 5 and 22in order to assure that the core and cladding will not come togetherearlier than at the bite of the rolls and in order to put as much shearas possible at the interface, The shear at the interface enables theprovision of at least 10% increased surface area than between the planmaterials, i.e., the interface between the core and cladding ischaracterized by a wave-like formation with a significant increasedinterfacial contact area. It is further noted that the interfacialsurface betwen the core and cladding is characterized by the absence ofapparent interatomic diffusion between the core and cladding material.For example, when an aluminum alloy cladding is used, there issubstantially no diffusion of aluminum atoms into the copper core and noapparent formation of deleterious Cu-Al intermetallic compounds.

A further advantage of the present invention is that subsequent to therolling operation the composites of the present invention do not requiresubsequent diffusion anneals, i.e., conventional processing frequentlyrequires diffusion anneals to secure the bond between core and cladding.The fact that the present invention does not require diffusion annealsis particularly significant since diffusion anneals might and frequentlydo cause blisters or the like due to the long treatment times requiredand co-diffusion of gases to the interface between the core andcladding. Diffusion anneals also promote the formation of deleteriousintermetallic compounds.

In fact, subsequent to the rolling operation no further operationswhatever are required. The composites of the present invention areprovided in commercial form ready to be used for the desiredapplication. It may naturally be desirable to perform conventionalsubsequent operations for particular applications. For example, shortthermal treatments for stress relief or the attainment of desiredproperties, e.g., a short heat treating anneal or aging treatment, arolling operation for dimensional control, additional work hardening,and so forth.

The present invention will be more readily understandable from aconsideration of the following illustrative examples.

EXAMPLE I In the following example the core was clad on both sides, thecore material was an electrolytic, commercial purity copper alloycontaining 99.9% copper in strip form having a gage of 0.160" and thecladding material was a copper base alloy containing about 75% copperand 25 nickel at a gage of 0.045". All of the processing of thesematerials was conducted in air.

Both the core and the claddings were in the fully annealed condition;therefore, both the core and claddings were in the recrystallizedcondition.

Both the core and claddings were cleaned in an alkaline detergentsolution, rinsed with warm water, dried with warm dry air and thesurfaces of the core and claddings abraded with a rotating wire brush.

The core and claddings were then rolled together at room temperature(approximately 78 F.) with no prior heating to either the core orcladdings. The speed of the rolls was 40150 feet per minute using acontinuous, variable speed rolling mill. The materials were rolled inone pass at a reduction of 80%. The included angle between claddings was12 with the core bisecting this. The core and claddings came togetherfor the first time in the bite of the rolls, with the claddingscontacting the rolls prior to contacting the core.

The resultant composite is shown in FIGURE 1 at a magnification of 65 X.The composite was characterized by having a moderate bond strength, witha peel strength of from 40 to 60 pounds. The peel strength is a measureof the strength of the bond and is determined on a A" wide specimenwherein the cladding is directly peeled from the core. The peel strengthis a measure of the force necessary to pull the composite apart. Thehigher the peel strength the better the bond.

The resultant composite was about 0.050" thick. Examination of FIGURE 1shows (1) there was no apparent interatomic diffusion between claddingsand core and (2) there was at least greater bonding area at the interface than with planar materials, with the intersurface beingcharacterized by a moderate wave-like formation. It is also apparentfrom an examination of FIGURE 1 that neither the core nor claddings wererecrystallized. This is apparent from the elongated grain structure inFIGURE 1 shown in both the core and claddings.

EXAMPLE II In this example the same core and claddings as Example I wereused, with the core being clad on both sides. The starting materialswere of the same gage as in Example I, with all processing beingconducted in air.

-Both the core and claddings were in the fully annealed condition andboth the core and claddings were cleaned, dried and abaded as in ExampleI.

After the cleaning and abrading, the core only was placed in a furnaceand heated to 250 F. The core and claddings were then rolled immediatelyafter the core exited from the furnace with the cladding materials beingrolled cold. The materials were rolled in one pass with the rollingvariables being the same as Example I.

The resultant composite is shown in FIGURE 2 at a magnification of 65 X.The composite was characterized by having a good bond strength, with apeel strength in excess of 90 pounds.

Examination of FIGURE 2 shows (1) there was no apparent interatomicdiffusion between claddings and core and (2) there was at least 10%greater bonding area at the interface than with planar materials, withthe intersurface being characterized by a moderate wave-like formation.It is also apparent from FIGURE 2 that the claddings were notrecrystallized while the core was recrystallized. It should be notedthat after heating the core to 250 F, he core was till in therecrystallized c ndition; however, in the bite of the rolls the corematerial was continually being cold worked and the reduction allowed therecrystallization process to start in the bite of the rolls.

EXAMPLE III Example I was repeated except that the core material wascold worked 11% prior to rolling; therefore, the core material was notrecrystallized as it entered the rolls.

The resultant composite is shown in FIGURE 3 at a magnification of 65X.The composite was characterized by having a good bond strength, with apeel strength in excess of pounds.

Examination of FIGURE 3 shows 1) there wa no apparent interatomicdiffusion between claddings and core and 2) there was at least 10%greater bonding area at the interface than with planar materials, withthe intersurface being characterized by a moderate wave-like formation.It is also apparent from FIGURE 3 that the claddings were notrecrystallized while the core was recrystallized. The core was notrecrystallized as it entered the rolls, but the 80% reduction taken wassufficient to cause recrystallization of the core material since thecore had been given 11% prior cold work.

EXAMPLE IV Example I was repeated with a core of super-purity aluminumand claddings of stainless steel. The aluminum core was in the coldworked, H14 temper. The core and claddings were rolled together as inExample I with a one pass reduction of 60%.

The resultant composite was characterized by good bond strength as inExamples II and III, with good physical properties comparable toExamples II and III. The claddings were not recrystallized, while thecore was recrystallized.

EXAMPLE V Example II and III were repeated, except that the claddingswere commercial cartridge brass having 70% copper and 30% zinc.

The resultant composites had characteristics comparable to those inExamples II and III with peel strengths in excess of 90 pounds. Thecladdings were not recrystallized, while the core was recrystallized.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning of the range of equivalency are intended to beembraced therein.

What is claimed is:

1. An integral composite article having a core clad with a dissimilarmetal, said article having a thickness less than 0.60 inch with theinterface between the core and cladding being characterized by theabsence of atomic interdiflusion, with said interface being furthercharacterized by having at least 10% greater surface contact area thanplanar sheets, with one component of the composite exhibiting amicrostructure of at least 25% equiaxed grains and another componenthaving grains grossly elongated in the rolling direction, with thelength to thickness ratio of said elongated grains being at least 2: 1,said core and cladding being in direct face-to-face contact.

2. A composite article according to claim 1 wherein said component witha microstructure of equaxed grains being the core.

3. A composite article according to claim 2 wherein said core is acopper base alloy.

4. A composite article according to claim 3 wherein said cladding iscupro-nickel.

(References on following page) References Cited UNITED G. K. WHITE,Assistant Examiner U.S. C1. X.R.

STATES PATENTS Boessenkool et a1. 29194 X Boessenkool et a1. 29194 XSiegel 29-194 X 5 Wardlaw 29497.5 X

3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,496,625 Dated February 24, 1970 Inventor(s) Joseph Winter It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[- In Column 1, line 48, the word "and" should read "are":

In Column 2, line 66, the word "sad" should read --said--,

In Column 3, line 64, the word "hat" should read --hot--.

In Column 4, line 16, the word "unrestrained" should read--unstrained--.

In Column 5, line 68, the word "it", second occurrence, should read--if--.

In Column 6, line 43, the word "plan" should read --planar--;

In Column 6, line 47, the word "betwen" should read --between--.

In Column 7, line 56, the word "abaded" should read --abraded--.

In Column 8, line 39, the word "Example" should read -Examp1es--;

In Column 8, line 68, the word "equaxed" should read --equiaxed--,

I. NW 1 1% Meat:

Edwardlimcmheglr. A E. SGHUYIIER, JR- uestmg m Oomissioner of Patents

